1. Field of Invention
Embodiments of the invention relate generally to, but are not limited to, systems and methods for pumping blood through the body, and more particularly, at least one embodiment is directed to a totally artificial heart and method of controlling the artificial heart, while other embodiments may be directed to devices and methods for assisting the operation of a patient's heart.
2. Discussion of Related Art
Total artificial hearts (TAH) utilizing positive displacement pumps have been successfully used to replace the functions of the human heart. See, for example, U.S. Pat. No. 4,888,011 to Kung et al. entitled “Artificial Heart” (incorporated herein by reference in its entirety). Such pumps utilize an alternating left-right pumping device with left and right pumping chambers, each including a membrane or diaphragm separating the chamber into a blood flow section and a hydraulic section. During left-side blood pump ejection, hydraulic fluid is pumped from the right hydraulic section through a hydraulic pump into the left hydraulic section, hereby expanding the left side membrane into the left blood pumping section to forcibly eject blood in the section. At the same time, removal of hydraulic fluid from the right side hydraulic section causes the right side membrane to contract, resulting in concurrent filling of the right side blood pump while left side ejection is taking place. To maintain physiologic right atrium pressure (RAP) and left atrium pressure (LAP) a separate hydraulic chamber is used to derate the right side stroke volume. The hydraulic flow is reversed for right side ejection and left side filling.
As is known, the volume of blood flow pumped by the left side of the heart is typically higher than that pumped by the right side of the heart. In one type of prior art artificial heart, to compensate for this imbalance, an atrial shunt is provided between the left atrium and the right atrium. While the use of the shunt may help to compensate for the imbalance, this scheme allows mixing of oxygenated blood with deoxygenated blood reducing the efficiency of oxygenation under certain physiologic circumstances.
While positive displacement pumps have been successful in mimicking the functions of the human heart, a number of difficulties associated with the use of these devices has prevented wide spread use. For example, positive displacement pumps currently in use tend to be too large in size to be used in patients with smaller thoracic cavities. In order to mimic physiologic pressures and beat rates, stroke volumes in the range of 50 to 100 cc are typically needed for the left and right ventricle. Thus, using typical existing devices, the pumping chambers must be large enough to pump this volume of blood in a single beat. In addition, an energy converter is typically used to drive the hydraulic fluid between the left and right chambers, thereby increasing the overall size of the device. As a result, such systems normally have a total volume in the range of 700 to 800 cc in order to provide an output of up to 8 L/min. Moreover, positive displacement pumps typically require the use of artificial valves in order to pump blood from the pumping chambers in a unidirectional manner.
One alternative approach to using positive displacement pumps is to use rotary pumps, which pump blood directly, rather than by displacement. These rotary pumps, which generally include axial and centrifugal flow pumps, can be operated in either steady or pulsatile flow modes, and do not require a stroke volume to be the determining factor with respect to the size of the system. By controlling the rotary pump rotor speed, pulsatile flow can be delivered from these pumps at physiologic volumes and pressures with system sizes not significantly larger than the stroke volumes delivered.
An example rotary pump of the centrifugal type is disclosed in U.S. Pat. No. 5,017,103 to Dahl entitled “Centrifugal Blood Pump and Magnetic Coupling” (which is hereby incorporated by reference). This pump has a broad, relatively flat impeller situated within a housing that has inlet and outlet tube connector ports. Another exemplary rotary pump is disclosed in U.S. Pat. No. 6,071,093 to Hart et al., entitled “Bearingless Blood Pump and Electronic Drive System” (which is hereby incorporated by reference). Hart discloses a rotary pump having a magnetically and/or hydrostatically suspended rotor.
While rotary pumps have the advantage of mechanical simplicity and small size, a number of issues must still be resolved before such pumps can be used in a total artificial heart. Among the issues that need to be addressed are control strategies that allow the pumps to respond to varying physiological demand, and that accommodate the natural flow imbalance between the pulmonary and systemic circulations.